Shoes are generally intended to provide comfort and protection to the foot by fulfilling a number of functions related to the interface between the bottom of the foot and the surface on which the foot impacts during walking and running. Among these functions are: protection against cuts and abrasion; traction to prevent slipping; shock absorption to avoid injuries and bone and muscle damage that can be caused by repeated pounding of the foot against the walking or running surface; flexibility to allow natural body movements; cushioning for comfort; and the ability to behave elastically so that energy is conserved in walking and running.
Running shoes are shoes specifically made for running. Some running shoes are made for athletic competitions based on speed and endurance. Other running shoes are made for training for said competitions, as well as for non-competitive-related running for purposes such as exercise and fun. It is desirable during periods of actual competition to maximize the elastic behavior of a running shoe each time the runner's foot hits the ground, so as to conserve energy and provide a spring-like energy-returning effect with each step the runner makes and thereby assist the runner in achieving and sustaining higher speed, while nevertheless giving a level of cushioning and energy absorption suitable for comfort and injury and damage prevention. However, when running shoes are worn during periods when higher speed is less important, such as non-competitive running, walking, and jogging, it is desirable to maximize cushioning for comfort and shock absorption to prevent injury and damage. Moreover, it is desirable that all components of a running shoe be durable and lightweight.
Elasticity affects speed in two important ways. First, when the shoe behaves elastically, more energy is returned, and running becomes more efficient. It is known from physics that the fundamental, or resonant, frequency (F) of simple harmonic oscillator (a mass connected to a spring) is given by the expression,F=A times square root (K/M)where A is a constant, K is elasticity of the spring, and M is the mass of the body. The amplitude of oscillation and energy efficiency is greatest at resonant frequency, and the above equation shows that the resonant frequency increases with increasing elasticity, and with decreasing weight. A runner's resonant frequency also increases in a similar way, so that as the shoes become more elastic, at a given weight the runner becomes more efficient at a faster pace.
According to Hooke's Law, elastic materials can be described in terms of a property known as the elastic modulus, that is, a linear relationship between applied force and the amount the materials deform. For a given level of applied force, low-modulus materials deform more than high-modulus materials. Running shoes that interpose low-modulus materials between the bottom of the foot and the walking and running surface are better for absorbing energy to provide cushioning and shock absorption. Running shoes that interpose high-modulus materials are better for storing elastic energy and returning it to the runner's foot as it lifts off the ground. Running shoes can be optimized for either cushioning and shock absorption on the one hand or speed on the other hand by control of the elastic modulus.
Accordingly, a great variety of running shoes and related devices is available on the market and described in prior art. Many running shoe components and materials are known which provide cushioning that attenuates and dissipates ground reaction forces. Prior art shoes have long incorporated a midsole composed of closed cell viscoelastic foams, such as ethyl vinyl acetate (“EVA”) and polyurethane (“PU”). EVA and PU are lightweight and stable foam materials that possess viscous and elastic qualities. The density or durometer, i.e., hardness, of EVA and PU can be altered by adjusting the manufacturing technique to provide differing degrees of cushioning. Alternate shoe structures for cushioning the impact of heel strike by incorporating gas or liquid or cushioning devices combinations thereof in chambers in the midsole are also well known. However, said running shoes and related devices are generally constructed of materials and in such a manner as to interpose materials having fixed elastic moduli between a runner's foot and the walking and running surface in order to achieve specific cushioning, shock absorbing and energy storing and returning properties.
Dilatant compounds are also well known. For purposes of this invention, a dilatant compound is a polymeric material that changes from soft and pliable under slow application of a load to elastic and bouncy under rapid application of a load. Technically, this means that a dilatant compound is a polymeric material whose yield point and elastic modulus increase with increasing strain rate. In other words, it is a liquid with inverse thixotropy, that is, a viscous liquid suspension that temporarily solidifies under applied pressure. Alternatively, it can be described as a liquid suspension in which the resistance to flow increases faster than the rate of flow.
A well-known example of a dilatant compound is the toy, Silly Putty® as described in U.S. Pat. No. 2,541,851. (Silly Putty is a registered trademark of Binney and Smith). Silly Putty® flows when slowly squeezed in the hand, but bounces when dropped on the floor. This behavior is known as strain-rate sensitivity. As shown in FIG. 7, the material is soft and pliable under slow application of load, or slow strain rate. At faster application of load, or high strain rate, the material behaves elastically, as indicated by the steeper slope of the left-hand side of the fast-load response shown schematically on FIG. 7.
Moreover, as shown in FIG. 7, the yield point, i.e., the load at which the response changes from sloped (elastic) to horizontal (plastic) also increases at faster application of load. Since the amount of elastic energy stored is equal to the area beneath the elastic portion of the curve, it is evident that much more energy is stored during fast loading.
While it has been taught to interpose devices having variable elastic moduli between a runner's foot and the midsoles of running shoes so as to provide variable shock absorbing and cushioning properties, it has not been taught to provide midsoles that achieve higher energy storing and returning properties at higher running speeds.